The present invention relates to a screw compressor for intermittent operation which comprises a compressor housing having a casing wall which encloses at least two compression chamber which partially overlap one another in a radial direction and each of which accommodates a screw rotor and is closed at the ends by a high-pressure end wall and a low-pressure end wall.
A compressor of this kind can be used conveniently for maintaining the level of pressure in the pneumatic brake system of an automotive vehicle. The compressor is normally activated, or engaged, via a coupling when the engine of the vehicle is started, and is de-activated or disengaged through the coupling subsequent to reaching the intended pressure in the brake system. The compressor is re-engaged when the need arises, i.e. when the pressure has fallen to a given level. The coupling required herefor is both relatively expensive and bulky, and can be omitted in those instances when the compressor is driven by a separate motor, such as an electric motor, capable of being started and stopped in a corresponding manner. This latter solution is not an ideal solution, however, and consequently there is a need for improvement in this regard.
The object of the present invention is to provide a compressor which is particularly suited to intermittent operation and which is not encumbered with the aforesaid drawbacks.
This object is achieved with a compressor constructed in accordance with the invention and having the characterizing features set forth in the claims.
By enabling the whole of the wall plate of the highpressure end wall located adjacent the rotor ends to be displaced axially through a short distance and therewith open, or unseal, the compression chambers, the air flowing through the compressor is recycled to the inlet side thereof practically without loss, i.e. the compressor is driven in a fully unloaded or inactive state until it needs to be re-engaged. Re-engagement of the compressor is effected by returning the wall plate of said high-pressure end wall to its chamber sealing position, this resetting of the wall plate preferably being achieved in a controlled manner, so as to initiate production of compressed air smoothly and evenly. The wall plate can be operated readily by means of an auxiliary piston device arranged in a cylinder chamber in the high-pressure end wall and connected to said wall plate. This arrangement solely requires a pressure fluid connection, and thus obviates the need for mechanical operating devices with associated lead-through bushings in the high-pressure end wall plate.
If the rotor bearings are positioned solely at the low-pressure end of the compressor, the ends of the rotors adjacent the high-pressure end thereof may be completely flat, thereby enabling the wall plate to be imperforate and also totally flat.
Since the compressor of this kind normally incorporates an oil injection facility, there is provided in accordance with one advantageous embodiment of the invention a valve arrangement which is contructed to curtail the inflow of oil, or alternatively to cut-off the oil supply completely, when the wall plate occupies its axially withdrawn or retracted position, corresponding to the inactive state of the compressor, since with the wall plate in this position it is not necessary to cool and seal the compression chambers. Neither is there any real need for lubrication when the compressor is in its inactive state.